1. Field of the Invention
The present invention relates generally to mechanical printing and more particularly to assemblies which contain, guide and advance a printing ribbon.
2. Description of the Prior Art
Currently U-shaped printing ribbon cartridge assemblies are widely used with impact printers. In such cartridges, a printing ribbon feeds from a ribbon feed aperture located at a projecting terminal end of an arm of the U-shaped cartridge. The ribbon thus exposed then follows a path across the open mouth of the U-shaped cartridge and reenters the cartridge at another location through a ribbon return aperture located in the projecting terminal end of its other arm. When installed in an impact printer, a print head moves back and forth along the length of ribbon spanning the open mouth of the U-shaped cartridge between the arms. Located on the opposite side of the exposed ribbon from the print head is the surface of a platen past which a web of material to be printed may pass. Thus arranged, motion of print head elements toward the platen cause points on the exposed length of printing ribbon to contact mark the web of material. Located within the base of the U-shaped cartridge, between the projecting arms and opposite to the open mouth, are a ribbon storage compartment and means for guiding and advancing a printing ribbon.
A printing ribbon spanning the open mouth of the cartridge is advanced by a pair of meshing toothed wheels located within the base toward the end closest to the point at which the ribbon reenters the cartridge. These wheels are adapted for rotation within the cartridge by circular, radial bearings formed about both of their respective terminal ends. These radial bearings are received and rotatably supported in the cartridge's walls by circular apertures at fixed locations. A ribbon contained within such a cartridge is guided from the point of return into the cartridge along a path passing between these wheels. Thus, by properly rotating one of the wheels, the ribbon may be advanced under tension across the open portion of the U-shaped cartridge. Thus far, desipite the fact that thin, high quality, single pass printing ribbons fabricated from solid film material are well known in the impact printing art, wire matrix impact printers having used cartridges containing only ribbon fabricated from thick, porous, woven material. Previously it has been impossible to use printing ribbon fabricated from solid film material in these cartridges either because they lack space to store a sufficient quantity of solid film material or because of the increased expense associated with having separate cartridge types for ribbons fabricated respectively from woven, porous fabric material and from solid film material. Generally woven ribbons are relatively thick having a thickness between 0.003 and 0.006 inches and film ribbons are relatively thin having a thickness between 0.0003 and 0.001 inches.
Since these cartridges have been only useful with woven ribbons and since such ribbons may be used repeatedly and because this cartridge structure does not permit reversing the direction of ribbon travel, ribbons used in these cartridges are formed into endless loops. Formed into such loops, the woven ribbon is guided from the meshing toothed wheels along a path passing through the base of the U-shaped cartridge to the point at which it is fed out. In order to increase the amount of ribbon which such a cartridge may contain, they are fabricated with a ribbon storage compartment within the base portion of the U-shape. The ribbon storage compartments are formed with two apertures positioned at opposite ends thereof. One aperture is located immediately adjacent to the toothed wheels to receive the ribbon immediately after it passes therebetween. The other aperture is located near the far end of the base of the "U" so that the ribbon may pass immediately to the cartridge exit point upon leaving the storage compartment.
Within the storage compartment, the ribbon follows a serpentine path back and forth across the compartment's width. The ribbon is arranged along this meandering path to permit the cartridge to store a much longer length of ribbon. To facilitate feeding the ribbon into and out of the compartment, its end walls are arranged in a V-shape pointing toward the respective apertures. To assure uniform feeding of the ribbon from its serpentine arrangement within the storage compartment, the walls of the compartment adjacent to the ribbon's edges are formed with a slightly raised rib crossing between the V-shaped walls pointing toward the ribbon's exit aperture. The opposing surfaces of these ribs are spaced at a distance across the storage compartment which is less than the width of the ribbon. Thus, in passing between these ribs, the ribbon's edges are frictionally engaged and the ribbon must bend slightly. Thus, these ribs constrain the ribbon's serpentine path to the region between the storage compartment's entry aperture and the location of these ribs. Along the ribbon's entire path outside of this region of the storage compartment, the ribbon is under tension until it reaches the toothed wheel. Therefore, the ribbon's path outside of this region is essentially planar except at locations in which it passes around the cartridge's ribbon guiding members.
Further, because both sides of a woven ribbon may be used for printing, the endless loops used in these cartridges are generally formed into a mobius strip; i.e. a continuous, closed loop having only a single surface which may be formed by joining opposite terminal ends of a length of flat, flexible material. Cartridges used with a mobius strip ribbon must contain a guide element along the ribbon's path which directs the ribbon along a path which reverses the cartridge walls to which its edges are adjacent. Thus, opposite side surfaces of a fixed location on a ribbon so formed and guided are alternately adjacent to the web to be printed when passing across the open mouth of the U-shaped cartridge. Because the mobius guide element frictionally engages the ribbon and therefore increaes the tension therein, it is usually located along the ribbon's path between the exit from the storage compartment and the point at which the ribbon feeds from the cartridge.